The invention relates to a method of processing an electronic signal representative of a sound for direct reproduction or for recording. The invention further relates to an apparatus for processing an electrical signal repesentative of a sound for direct reproduction or for recording.
Direct reproduction of picked-up sound is employed, for example, in live radio broadcasts in which sound is picked up and directly transmitted. Recording of picked-up sound is effected, for example, on grammophone records or audio tapes. A problem inherent in presently known sound recording techniques is that the sound cannot be recorded so that the listener obtains an optimal spatial impression of the recorded sound when it is reproduced. In so-called stereophonic systems employing at least two separate microphones or microphone systems for sound recording and at least two separate channels for sound reproduction, a certain spatial impression is created indeed. This spatial impression is even better when using the recording technique described in U.S. Pat. No. 4,074,084, in which for recording sound at least two microphone systems of the dummy head type are used, signals from corresponding microphones of the dummy heads being coupled with each other and, when reproducing the recorded sound, being reproduced in one of the two channels. Although in this manner a considerable enhancement of the sound reproduction is achieved, it remains difficult and even impossible to realize such a sound reproduction that the listener is able to at all times distinguish directions in the sound, such as in front and behind, above and below, left and right.
It is an object of the invention to provide a solution to the above problem and to provide a method and apparatus for sound recording or for processing recorded sound prior to its reproduction so that a truly directional sensation is experienced during the reproduction.
It has long been known that the variation of amplitude pressure response of the normal human auditory system versus frequency is different for different angles of incidence of sound on the listener. These differences are apparent in the curves of sound pressure response of the ear canal versus frequency for different angles of incidence depicted in FIG. 6. These curves are taken from the article appearing in the publication Funkschau, 1974 Heft 11, pages 399-402. For example, the topmost one of these curves, denominated ".theta.=0", shows that for sound impinging on the listener from directly in front of him, the sound pressure response of the ear canal is relatively flat from about 4000 hertz to about 7000 hertz, with the response at 4000 through about 7000 hertz being about 9 decibels lower than the response at about 2200 hertz. When sound of equal sound pressure for all frequencies impinges on the listener from directly in front of him, the sound pressure produced in the ear canel by the 2200 hertz component will be greater than the sound pressure produced by the 4000 and 7000 hertz components and the sound pressures produced by each of these latter components will be approximately equal. By contrast, the next curve (denominated ".theta.=45.degree.") shows that the sound pressure response curve is markedly different for sounds impinging on the listener from an angle of about 45.degree.. If sound of equal sound pressure at all frequencies impinges on a listener at an angle of about 45.degree., the component at about 2300 hertz will produce the greatest sound pressure in the ear canal. The component at about 4000 hertz will produce a sound pressure about 9 decibels less than that produced by the component at about 2300 hertz. The component at about 7000 hertz will produce a sound pressure almost equal to that produced by the component at 2300 hertz and about 8 decibels greater than that produced by the component at about 4000 hertz.
The present invention results from the realization that a human being can use the differences in amplitude response as cues for determining where a source of sound is located. For example, if a person is directly facing a piano so that the sound of the piano is impinging upon him from directly in front of him, and the piano simultaneously produces notes of equal amplitude at 2200 hertz, at 4000 hertz and 7000 hertz, the sound pressure level in the listener's ear produced by the 2200 hertz note will be greater than that produced by the 4000 hertz note or the 7000 hertz note, and the sound pressure levels produced by the 4000 hertz note and 7000 hertz note will be approximately equal to one another. The listener does not regard this sound as distorted; he perceives all of these notes as being of equal amplitude. If the piano then moves to a location 45.degree. to the left of the listener, and repeats this same set of notes, the sound pressure levels produced in the listener's ear canal will be different. The notes at 2200 and 7000 hertz will produce almost equal levels of sound pressure, while the note at 4000 hertz will produce a level of sound pressure distinctly lower than either of the other two notes. The listener, however, still does not regard the sound as distorted, even though the relative sound pressure levels produced by the three notes of the set have changed markedly. Rather, the listener still recognizes all of the notes as being of equal amplitude but now believes that they emanate from a source at 45.degree. to him. In short, the listener is cued by the difference in distortion to recognize a difference in direction.
The present invention results from the further realization that, if an electronic signal representative of a sound is processed to introduce into it amplitude distortions corresponding to the distortions introduced by the human auditory system for sounds impinging on the listener from a preselected direction, and the electronic signal is then converted into sound, the response of the listener's auditory system will vary according to frequency as though the sound were impinging upon the listener from the preselected direction. The listener will believe that the sound does impinge upon him from such preselected direction even if it is, in fact, impinging on him from some other direction. The listener will be unable to tell whether the differences in the amplitudes of the responses were induced by the natural distortion caused by his anatomy or by artificial distortion introduced electronically.
Thus, in the method of the present invention, directional information is added to an audio frequency electronic signal including components of various frequencies by differentially adjusting the amplitudes of the various components.
In the method of the present invention, an electronic signal representative of a sound is processed, prior to its reconversion into sound. In such processing, the signal is modulated with a "modulating signal" representative of a specific, preselected direction of perception, so that when the electronic signal is subsequently reconverted into sound, the sound will be perceived as seemingly emanating from a source disposed in such preselected direction from the listener.
As used herein, the verb "modulate" should be understood as meaning "to impress information on a signal". Also, the term "signal" should be understood as meaning "information or intelligence". Thus, in the method of the present invention, various amplitude adjustments are applied to various components of the electronic signal. These amplitude adjustments vary with frequency in a manner corresponding to the variation of amplitude response with frequency of the normal auditory system for sounds impinging on the listener from a preselected direction of perception. Thus, the pattern of amplitude adjustments constitutes the intelligence or modulating signal which is impressed upon the original electronic signal.
The method of the present invention may be applied to an electronic signal representative of a sound signal either in a sound reproduction method wherein the electronic signal is reconverted into sound substantially simultaneously with its formation, or in a method of sound reproduction in which the electronic signal is recorded. For example, the method of the present invention may be applied in live radio broadcasting to process an electronic signal which is substantially simultaneously broadcast, received by a radio receiver, and reproduced into sound, and the method of the present invention may be applied to process an electronic signal which is recorded and then reproduced into sound at some later time. The method of the present invention may be applied to process a single electronic signal by modulating it with a modulating signal representative of a single preselected direction of perception, and the method of the present invention may also be applied to simultaneously process a plurality of electronic signals by modulating each of such signals with a different modulating signal representative of a different preselected direction of perception.
The plural signal processing methods of the present invention preferably include the step of creating the electronic signals to be processed by use of a plurality of directional microphones, each adapted to receive sound predominantly from a specific preselected reception direction. The electronic signal created by each such microphone is separately processed to modulate it with a modulating signal characteristic of a preselected direction of perception corresponding to the reception direction of such microphone. When a plurality of microphones is used these microphones are preferably mounted relative to each other so that they are located on an imaginary or real spherical surface, and so that the reception direction of each microphone corresponds with its location on the spherical surface.
The present invention also includes apparatus for modulating an electronic signal with the manner described above. Such apparatus includes a "direction characteristic forming unit" which is capable of modulating an electrical signal with a modulating signal representative of a preselected direction of perception, to thereby apply varying amplitude adjustments to different portions of such electrical signal.
The present invention also includes a method of determining the variation in response with frequency of the human auditory system for sounds impinging on the listener from any direction. This method of measurement can be utilized to compile more accurate data to supplement and correct the previously known data.
By employing a direction characteristic forming or introducing element or elements, in accordance with the invention all the desired corrections with respect to phenomena affecting the direction of perception can be applied to an electronic signal representative of a sound prior to reconversion of such signal into sound. Additional correction (modulation of the electronic signal with a further modulating signal) may be useful if the signal is to be reproduced into sound through loudspeaker boxes. When reproducing through loudspeaker boxes, the position of these boxes relative to the listener will introduce a further, undesired direction characteristic into the sound. This can be compensated for in advance by further modulating the signal to be reproduced so that such undesired direction characteristic is nullified. Thus, if the electrical signal is to be reproduced into sound by a loudspeaker disposed in a loudspeaker direction from the listener, the signal should be additionally modulated with a signal comprising a pattern of amplitude adjustments which is the inverse of the direction characteristic of such loudspeaker direction. If desired, for this purpose the reproducing means themselves (in this case the loudspeaker boxes) may include an element providing the desired inverse characteristic.
The method according to the invention may be combined to advantage with the method described in the above U.S. patent. In that case, a method of recording sound is achieved in which the sound is picked up by means of at least two microphone systems each, in accordance with the invention, coupled in one way or another with direction characteristic forming elements, the signals originating from each microphone of a system being coupled, prior to or after their modulation, with the signals originating from the corresponding microphone or microphones of the other system or the other systems.